Create a metabox with multiple fields

metabox-multiple.php

<?php

/**
 * Create a metabox with multiple fields.
 * Replace `_namespace` with some namespace for your project to avoid conflicts with other items
 */

	//
	// Create Metabox
	//

	/**
	 * Create the metabox
	 * @link https://developer.wordpress.org/reference/functions/add_meta_box/
	 */
	function _namespace_create_metabox() {
		// Can only be used on a single post type (ie. page or post or a custom post type).
		// Must be repeated for each post type you want the metabox to appear on.
		add_meta_box(
			'_namespace_metabox', // Metabox ID
			'Some Metabox', // Title to display
			'_namespace_render_metabox', // Function to call that contains the metabox content
			'post', // Post type to display metabox on
			'normal', // Where to put it (normal = main colum, side = sidebar, etc.)
			'default' // Priority relative to other metaboxes
		);
	}
	add_action( 'add_meta_boxes', '_namespace_create_metabox' );



	/**
	 * Create the metabox default values
	 * This allows us to save multiple values in an array, reducing the size of our database.
	 * Setting defaults helps avoid "array key doesn't exit" issues.
	 * @todo
	 */
	function _namespace_metabox_defaults() {
		return array(
			'item_1' => 'Some value',
			'item_2' => 'on',
			'item_3' => 5,
		);
	}



	/**
	 * Render the metabox markup
	 * This is the function called in `_namespace_create_metabox()`
	 */
	function _namespace_render_metabox() {

		// Variables
		global $post; // Get the current post data
		$saved = get_post_meta( $post->ID, '_namespace', true ); // Get the saved values
		$defaults = _namespace_metabox_defaults(); // Get the default values
		$details = wp_parse_args( $saved, $defaults ); // Merge the two in case any fields don't exist in the saved data

		?>

			<fieldset>

				<?php
					// A simple text input
				?>
				<div>
					<label for="_namespace_custom_metabox_item_1">
						<?php
							// This runs the text through a translation and echoes it (for internationalization)
							_e( 'Item 1', '_namespace' );
						?>
					</label>
					<?php
						// It's important that the `name` is an array. This let's us
						// easily loop through all fields later when we go to save
						// our submitted data.
						//
						// The `esc_attr()` function here escapes the data for
						// HTML attribute use to avoid unexpected issues
					?>
					<input
						type="text"
						name="_namespace_custom_metabox[item_1]"
						id="_namespace_custom_metabox_item_1"
						value="<?php echo esc_attr( $details['item_1'] ); ?>"
					>
				</div>


				<?php
					// A simple text input
				?>
				<div>
					<label>
						<?php _e( 'Item 2', '_namespace' ); ?>
						<input
							type="checkbox"
							name="_namespace_custom_metabox[item_2]"
							value="1"
							<?php
								// `checked()` compares two values, and if they match
								// echoes `checked="checked"`, checking the checkbox
								checked( $details['item_2'], 'on' );
							?>
						>
					</label>
				</div>


				<?php
					// A numeric input
				?>
				<div>
					<label for="_namespace_custom_metabox_item_3">
						<?php _e( 'Item 3', '_namespace' ); ?>
					</label>
					<input type="number" name="_namespace_custom_metabox[item_3]" id="_namespace_custom_metabox_item_3" value="<?php echo esc_attr( $details['item_3'] ); ?>">
				</div>

			</fieldset>

		<?php

		// Security field
		// This validates that submission came from the
		// actual dashboard and not the front end or
		// a remote server.
		wp_nonce_field( '_namespace_form_metabox_nonce', '_namespace_form_metabox_process' );

	}



	//
	// Save our data
	//

	/**
	 * Save the metabox
	 * @param  Number $post_id The post ID
	 * @param  Array  $post    The post data
	 */
	function _namespace_save_metabox( $post_id, $post ) {

		// Verify that our security field exists. If not, bail.
		if ( !isset( $_POST['_namespace_form_metabox_process'] ) ) return;

		// Verify data came from edit/dashboard screen
		if ( !wp_verify_nonce( $_POST['_namespace_form_metabox_process'], '_namespace_form_metabox_nonce' ) ) {
			return $post->ID;
		}

		// Verify user has permission to edit post
		if ( !current_user_can( 'edit_post', $post->ID )) {
			return $post->ID;
		}

		// Check that our custom fields are being passed along
		// This is the `name` value array. We can grab all
		// of the fields and their values at once.
		if ( !isset( $_POST['_namespace_custom_metabox'] ) ) {
			return $post->ID;
		}

		/**
		 * Sanitize all data
		 * This keeps malicious code out of our database.
		 */

		// Set up an empty array
		$sanitized = array();

		// Loop through each of our fields
		foreach ( $_POST['_namespace_custom_metabox'] as $key => $detail ) {
			// Sanitize the data and push it to our new array
			// `wp_filter_post_kses` strips our dangerous server values
			// and allows through anything you can include a post.
			$sanitized[$key] = wp_filter_post_kses( $detail );
		}

		// Save our submissions to the database
		update_post_meta( $post->ID, '_namespace', $sanitized );

	}
	add_action( 'save_post', '_namespace_save_metabox', 1, 2 );



	//
	// Save a copy to our revision history
	// This is optional, and potentially undesireable for certain data types.
	// Restoring a a post to an old version will also update the metabox.

	/**
	 * Save events data to revisions
	 * @param  Number $post_id The post ID
	 */
	function _namespace_save_revisions( $post_id ) {

		// Check if it's a revision
		$parent_id = wp_is_post_revision( $post_id );

		// If is revision
		if ( $parent_id ) {

			// Get the saved data
			$parent = get_post( $parent_id );
			$details = get_post_meta( $parent->ID, '_namespace', true );

			// If data exists and is an array, add to revision
			if ( !empty( $details ) && is_array( $details ) ) {
				// Get the defaults
				$defaults = _namespace_metabox_defaults();

				// For each default item
				foreach ( $defaults as $key => $value ) {
					// If there's a saved value for the field, save it to the version history
					if ( array_key_exists( $key, $details ) ) {
						add_metadata( 'post', $post_id, '_namespace_' . $key, $details[$key] );
					}
				}
			}

		}

	}
	add_action( 'save_post', '_namespace_save_revisions' );



	/**
	 * Restore events data with post revisions
	 * @param  Number $post_id     The post ID
	 * @param  Number $revision_id The revision ID
	 */
	function _namespace_restore_revisions( $post_id, $revision_id ) {

		// Variables
		$post = get_post( $post_id ); // The post
		$revision = get_post( $revision_id ); // The revision
		$defaults = _namespace_metabox_defaults(); // The default values
		$details = array(); // An empty array for our new metadata values

		// Update content
		// For each field
		foreach ( $defaults as $key => $value ) {

			// Get the revision history version
			$detail_revision = get_metadata( 'post', $revision->ID, '_namespace_' . $key, true );

			// If a historic version exists, add it to our new data
			if ( isset( $detail_revision ) ) {
				$details[$key] = $detail_revision;
			}
		}

		// Replace our saved data with the old version
		update_post_meta( $post_id, '_namespace', $details );

	}
	add_action( 'wp_restore_post_revision', '_namespace_restore_revisions', 10, 2 );



	/**
	 * Get the data to display on the revisions page
	 * @param  Array $fields The fields
	 * @return Array The fields
	 */
	function _namespace_get_revisions_fields( $fields ) {

		// Get our default values
		$defaults = _namespace_metabox_defaults();

		// For each field, use the key as the title
		foreach ( $defaults as $key => $value ) {
			$fields['_namespace_' . $key] = ucfirst( $key );
		}

		return $fields;

	}
	add_filter( '_wp_post_revision_fields', '_namespace_get_revisions_fields' );



	/**
	 * Display the data on the revisions page
	 * @param  String|Array $value The field value
	 * @param  Array        $field The field
	 */
	function _namespace_display_revisions_fields( $value, $field ) {
		global $revision;
		return get_metadata( 'post', $revision->ID, $field, true );
	}
	add_filter( '_wp_post_revision_field_my_meta', '_namespace_display_revisions_fields', 10, 2 );

This is great! Though I'm having trouble figuring out how to display specific values on the front end.

How do you get the defaults and existing meta values array merge to work? When I retrieve meta values, I get an array of arrays, and can't figure out how to merge them properly.

Your code:
$saved = get_post_meta( $post->ID, '_namespace', true ); // Get the saved values
$defaults = _namespace_metabox_defaults(); // Get the default values
$details = wp_parse_args( $saved, $defaults ); // Merge the two in case any fields don't exist in the saved data

Using print_r() I can see that the meta values are returned as an array of arrays:
array(6) { ["title"]=> array(1) { [0]=> string(31) "Coordinator" } ["startdate"]=> array(1) { [0]=> string(4) "2018" } ["enddate"]=> string(0) "" ["priority"]=> array(1) { [0]=> string(1) "3" } ["_edit_lock"]=> array(1) { [0]=> string(12) "1534262084:1" } ["_edit_last"]=> array(1) { [0]=> string(1) "1" } }

$saved = get_post_meta( get_the_ID(), '_namespace', true );
echo $saved['item_1'];
echo $saved['item_2'];
echo $saved['item_3'];

This works for me to get the value